1. Field of the Invention
This invention relates to heating furnaces. In one of its aspects, the invention relates to a method and apparatus for controlling the flow of gases through two sides of a ceramic heat exchanger which is used in conjunction with an industrial heating furnace.
2. State of the Prior Art
Industrial heating furnaces are used for heating billets, for drying paint and solvents from lithograph sheets and for many other processes. In these types of operations, the articles are usually passed through the furnace from an entrance end to an exit end. As the articles pass through the furnace they are subjected to heated air which raises the temperature of the articles to a desired temperature. Typically, the heat is supplied through burners which burn fuel with preheated air. It has recently been disclosed in copending U.S. Pat. application Ser. No. 621,302 filed Oct. 10, 1975, now U.S. Pat. No. 4,022,571, to heat exchange the hot exhaust gases from the furnace with incoming air by means of a rotary ceramic heat exchanger.
The heat requirements for the furnace change from time to time. For example, sometimes the industrial line is closed down temporarily and the flow of articles through the furnace ceases. Under such conditions, the heat requirements to the furnace are drastically diminished. In order to prevent overheating, the heat supplied to the furnace is decreased to maintain the temperature within predetermined limits. In order to decrease the heat supplied, the air and fuel supplied to the burners are decreased.
The decrease in the supply of air to the furnace causes a change in the amount of air passed through the heat exchanger. With less air flowing through the heat exchanger, the ceramic heat exchanger is subjected to greater thermal gradients and thermal cycles which deleteriously affect the life of the ceramic heat exchanger. It is thus desirable to maintain the thermal gradient across the heat exchanger as constant as possible and to minimize cyclical thermal fluctuations in the heat exchanger.